1. Field of the Invention
The present invention relates to high energy, non-aqueous electrolyte based electrochemical energy storage devices such as high energy density batteries or high power electrochemical capacitors which are non-flammable. More particularly, this invention relates to non-flammable high energy, non-aqueous electrolyte based electrochemical energy storage devices containing an electrolyte solution including alkyl phosphate, which afford protection of the electrodes from the electrolyte at wide temperature ranges, and reduce ignition risks.
2. Discussion of the Prior Art
High voltage and high energy density rechargeable batteries based on non-aqueous electrolyte solutions are widely used as electric sources for various types of consumer electronic appliances, such as camcorders, notebook computers, and cell phones, because of their high voltage and high energy density as well as their reliability such as storage characteristics. This type of battery conventionally employs the complexed oxides of lithium and a transition metal as positive electrode, such as LiCoO2, LiNiO2, LiMn2O4, and variations of the previous oxides with different dopants and different stoichiometry, and additionally includes lithium metal, lithium alloys and/or carbonaceous materials as a negative electrode. Chosen over the lithium metal and lithium alloys are carbonaceous negative electrode materials, which are in general partially or fully graphitized and specially modified natural graphite. When a carbonaceous negative electrode is used, this battery is often referred to as a lithium-ion (Li-ion) battery, because no pure lithium metal is present in the negative electrode. During charge and discharge processes, the lithium ions are intercalated into and de-intercalated from the carbonaceous negative electrode, respectively. The advantages of using these negative electrodes is that problems associated with growth of lithium metal dendrites is avoided. Such dendrites are often observed in lithium or lithium alloy negative electrodes, and is known to cause short-circuiting of the cells.
Battery manufacturers have been trying to replace the widely used electrolyte solvent ethylene carbonate (EC) with propylene carbonate (PC), because the latter is cheaper and also improves low-temperature performance. However, the high power negative electrode material graphite is known to disintegrate in presence of PC (a process called “exfoliation”), and accordingly destroys battery performance.
It is recognized that the Li-ion cells work well in the carbonate solvent systems. This is due to the protection film formation on the carbon electrodes in the carbonate solvent system, as described in Koshina et al., “Relationship between Electrolyte and Graphite Electrode in Lithium Ion Batteries,” Proc. 1st Hawaii Battery Conference, Big Island of Hawaii, 5-7 Jan. 1998, herein incorporated by reference in its entirety. Upon the first charge of the lithium ion cell, the electrolyte decomposes at both anode and cathode, and the consequent decomposition product forms a dense film covering the aforementioned electrode surface. This film is permeable only to migrating lithium ions, but insulating to electrons. Therefore, the stability of this protection film constitutes the foundation on which the lithium ion battery functions. The ability of this film to resist dissolution into the carbonate-based solvents at different temperatures is a key-factor to the capacity retention and life cycle of the cell under different application environments. The carbonate-based electrolyte works well in a temperature range from −20 to 50° C. Recent studies of the liquidus temperature ranges of the binary carbonate systems, such as Ding et al., “Liquid-Solid Phase Diagrams of Binary Carbonates for Lithium Batteries,” J. Electrochem. Soc., 147 (5), 1688 (2000), herein incorporated by reference in its entirety, have shown that, the electrolyte freezes at temperatures below −20° C. To extend the temperature range to lower temperatures, it has been suggested, as described in Plichta et al., “Low Temperature Electrolyte for Lithium and Lithium-Ion Batteries,” Proc. 38th Power Sources Conference, p. 444, Cherry Hill, N.J., 8-11 Jun. 1998, and Smart et al., “Development of High Conductivity Lithium-ion Electrolytes for Low Temperature Cell Applications,” Proc. 38th Power Sources Conference, p. 452, Cherry Hill, N.J., 8-11 Jun. 1998 (each of which is hereby incorporated by reference in its entirety) to employ ternary solvent systems. As shown by Plichta et al., this approach works for a pure Li metal anode but it does not work properly for carbonaceous anode due to the high cell impedance involved at sub-ambient temperatures. This is because the film formed on the pure Li metal is different from that formed on the carbon surfaces.
Capacity loss as a result of high temperature storage and capacity fading when the cell is cycled at elevated temperatures are also problems in known Li-ion batteries. The protection layer that is believed to be formed in a mixture of one or two cyclic carbonates and one or more linear carbonates containing a lithium salt becomes eventually unstable at elevated temperatures, such that new films must be formed in each charging cycle, causing progressively fading capacity.
It is therefore desirable to have an electrolyte that can react with electrodes upon charge and form a better protection layer on either the anode or the cathode than those are formed by the mixture of cyclic and linear carbonates. It is more advantageous to have an electrolyte that can form better protection layers on both electrodes simultaneously. An electrolyte co-solvent, such as an organofluorine-containing compound, which has a C—F covalent bond that is both chemically and electrochemically inert and that will stay in the protection film after the compound is electrochemically decomposed on either electrodes, can potentially provide the best reaction product for the protection film on electrodes in highly oxidative and reductive reaction environment of Li batteries and Li-ion batteries, because organofluorine compounds are more resistant to salvation by carbonate solvent systems, in addition to its chemical/electrochemical stability.
The efforts of searching for such an electrolyte along this line of thought have been reported by others. For example, Nakano et al. in U.S. Pat. No. 5,750,730, herein incorporated by reference in its entirety, provided a fluorine-containing dioxolane compound is provided to improve charge and discharge cycle life of lithium rechargeable batteries or electric double layer capacitors. Besenhard et al. in U.S. Pat. No. 5,916,708, herein incorporated by reference in its entirety, provided partially fluorinated ethers that can enhance the safety of rechargeable lithium cells. Yokoyama et al. in U.S. Pat. No. 6,010,806, herein incorporated by reference in is entirety, provided fluoromethyl ethylene carbonate, which is excellent in voltage and charge and discharge cycle characteristics. These fluorinated compounds are very specific in that both the location and the degree of fluorination must be elaborately chosen so that the properties of the formed protection films can afford the expected properties. However, each of these systems exhibit problems related to flammability and ignition of the electrolyte solution.
Alkyl phosphates as a class of compound have been considered as flame retardants for lithium/lithium ion cells as is disclosed by Yokoyama et al in U.S. Pat. No. 5,580,684 and Narang et al in U.S. Pat. No. 5,830,600, both of which are incorporated by reference in their entireties. However, as will be described herein, the resulting electrolytes and electrolyte cells nevertheless are, in fact, flammable. Yokoyama et al. attempts to solve the flammability problem by providing a self-extinguishing electrolyte, which does initially ignite. Therefore, although the risk of flammability is reduced by the electrolyte of Yokoyama et al., there is nevertheless, ignition and the potential for flare-ups.